A conventional laser beam irradiation apparatus includes a laser beam generation means, an incident optical unit, an optical fiber, and an emission optical unit, as an example. When a laser beam such as pulse YAG laser beam is emitted from the laser beam generation means, the laser beam is concentrated by the incident optical unit, is then incident on the input side face of the optical fiber connected to the incident optical unit. The optical fiber includes a core having a circular cross section, and the laser beam is transmitted toward the output side face of the optical fiber while being reflected along the core thereinside. When the laser beam arrives at the output side face of the optical fiber, the laser beam is emitted from the output side face, and the laser beam is applied to a predetermined irradiation portion of a target to be irradiated through the emission optical unit. The predetermined irradiation portion means a welding portion if the conventional laser beam irradiation apparatus is a YAG laser welding machine. If the conventional laser beam irradiation apparatus is a laser beam processing apparatus, the predetermined irradiation portion means a surface metal layer or a point where a thin film formed on a surface has to be removed (see Patent Document 1, paragraphs 0014 and 0024, FIGS. 1, 4, and 5).
Patent Document 1; Japanese Patent Laid-open Publication No. 2004-276108